Method for preparation of tetrafluoroethylene/hexafluoropropylene copolymer

ABSTRACT

A method for preparing a tetrafluoroethylene/hexafluoropropylene copolymer by polymerizing tetrafluoroethylene and hexafluoropropylene in an aqueous medium using a di(fluoroacyl) peroxide as a polymerization initiator characterized in that a fluoroalkylcarboxylic acid is added gives a stable molecular weight of the tetrafluoroethylene/hexafluoropropylene copolymer and an increased utilization efficiency of the polymerization initiator.

FIELD OF THE INVENTION

The present invention relates to a method for preparing atetrafluoroethylene/hexafluoropropylene copolymer.

RELATED ART

In the preparation of a tetrafluoroethylene/hexafluoropropylenecopolymer, usually an alkali, for example, sodium bicarbonate (NaHCO₃)is added to an aqueous medium before the initiation of thepolymerization in order to prevent the corrosion of a polymerizationreactor so that an aqueous alkaline solution is formed, and then a largeamount of a diperoxide initiator is added to conduct the polymerization.The reason why the alkali is added is that an acidic material such ashydrofluoric acid is prepared by the decomposition of the monomer duringthe polymerization so that the acidic material may corrode thepolymerization reactor.

However, since the polymerization initiator tends to be easilyhydrolyzed in the aqueous alkaline solution at the beginning of thepolymerization, the polymerization initiator cannot be effectivelyutilized.

In addition, pH of the aqueous medium varies widely so that thehydrolysis conversion is unstable. Accordingly, the initiator efficiencyis not constant and a resultant copolymer has an unstable molecularweight.

SUMMARY OF THE INVENTION

An object of the present invention is to stabilize the molecular weightof the tetrafluoroethylene/hexafluoropropylene copolymer prepared by thepolymerization in the aqueous medium using the di(fluoroacyl) peroxidepolymerization initiator so that the utilization efficiency of thepolymerization, initiator is increased.

The present invention provides a method for preparing atetrafluoroethylene/hexafluoropropylene copolymer by polymerizingtetrafluoroethylene and hexafluoropropylene in an aqueous medium using,as a polymerization initiator, a di(fluoroacyl) peroxide of the formula:##STR1## wherein X¹ and X² each is a hydrogen, fluorine or chlorineatom, m is an integer of 4 to 8 when X¹ is the hydrogen atom, and m isan integer of 3 to 8 when X¹ is the fluorine or chlorine atom, and n isan integer of 4 to 8 when X² is the hydrogen atom, and n is an integerof 3 to 8 when X² is the fluorine or chlorine atom, characterized inthat a fluoroalkylcarboxylic acid of the formula: ##STR2## wherein Y¹ isa hydrogen, fluorine or chlorine atom, and p is an integer of 4 to 8when Y¹ is the hydrogen atom, and p is an integer of 3 to 8 when Y¹ isthe fluorine or chlorine atom, is added to a polymerization systembefore the initiation of the polymerization or during thepolymerization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relationship between a reaction time and pHduring the copolymerization of tetrafluoroethylene andhexafluoropropylene.

DETAILED DESCRIPTION OF THE INVENTION

The Y¹ --C_(p) F_(2p) -- group in the fluoroalkylcarboxylic acid may belinear or branched. Specific examples of the fluoroalkylcarboxylic acidare ω-hydroperfluoroheptanoic acid, ω-hydroperfluorononanoic acid,ω-hydroperfluoroheptanoic acid, ω-hydroperfluoropentanoic acid,perfluoropropionic acid, perfluorobutanoic acid, perfluropentanoic acid,perfluorohexanoic acid, perfluoroheptanoic acid, perfluorooctanoic acid,perfluorononanoic acid and the like. Preferably, the amount of the usedfluoroalkylcarboxylic acid is from 0.05 to 0.5 parts by weight per 100parts by weight of water in the polymerization system.

The X¹ --C_(m) F_(2m) -- group and the X² --C_(n) F_(2n) group in thedi(fluoroacyl) peroxide used as the polymerization initiator may belinear or branched and may be the same or different. Specific examplesof the di(fluoroacyl)peroxide are di(ω-hydro-dodecafluoroheptanoyl)peroxide, di(ω-hydro-tetradecafluorooctanoyl) peroxide,di(ω-hydro-hexadecafluorononanoyl) peroxide, di(perfluorobutyryl)peroxide, di(perfluorovaleryl) peroxide, di(perfluorohexanoyl) peroxide,di(perfluoroheptanoyl) peroxide, di(perfluorooctanoyl) peroxide,di(perfluorononanoyl) peroxide, di(ω-chloro-hexafluorobutyryl) peroxide,di(ω-chloro-decafluorohexanoyl) peroxide,di(ω-chloro-tetradecafluorooctanoyl) peroxide,ω-hydro-dodecafluoroheptanoyl-ω-hydro-hexadecafluorononanoyl peroxide,ω-chloro-hexafluorobutyryl-ω-chloro-decafluorohexanoyl peroxide,ω-hydro-dodecafluoroheptanoyl-perfluorobutyryl peroxide and the like.

In the present invention, the amount of the used polymerizationinitiator is suitably determined according to the molecular weight ofthe copolymer and the contents of tetrafluoroethylene (referred to as"TFE" hereinafter) and hexafluoropropylene (referred to as "HFP"hereinafter) in the copolymer. Usually it is from about 0.01 to about 1parts by weight per 100 parts by weight of the monomer. Since thepolymerization initiator has a relatively high decomposition rate andthe concentration of the polymerization initiator tends to decreaseconsiderably during the polymerization reaction, if necessary, thepolymerization initiator is suitably added to the polymerization systemduring the polymerization reaction.

Preferably, the Y¹ --C_(p) F_(2p) -- group in the fluoroalkylcarboxylicacid is the same as one, preferably both, of the X¹ --C_(m) F_(2m) --group and the X² --C_(n) F_(2n) -- group in the di(fluoroacyl) peroxide.

The polymerization reaction is initiated by charging pure water and thefluoroalkylcarboxylic acid in a pressure vessel, adding TFE and HFP andthen adding the polymerization initiator. The ratio by volume of waterto the monomer is preferably from about 1:1 to about 10:1, particularlyabout 2:1 to about 5:1. The content of TFE in the TFE/HFP monomermixture is determined according to the composition of the objectivecopolymer. There is the following relationship at about 20°-25° C.:

    ______________________________________                                        The content of TFE                                                                              The content of TFE                                          in the monomer mixture (%)                                                                      in the copolymer (%)                                        ______________________________________                                         5                78                                                          10                87                                                          20                92                                                          ______________________________________                                    

Water is used as the reaction medium. If necessary, a solvent in whichTFE and HFP dissolve may be added in a suitable amount. Preferably,specific examples of such solvent are fluorine-containing solvents suchas trichlorotrifluoroethane, monofluorotrichloromethane and the like. Ifnecessary, any of an emulsion stabilizing agent such as polyvinylalcohol and sodium carboxymethylcellulose, sodium bicarbonate and boraxmay be added.

A polymerization temperature is restricted mainly by the decompositionrate, of the polymerization initiator. The decomposition reaction of thepolymerization initiator in the presence of water proceeds as afirst-order reaction and a rate constant (k) of the reaction is, forexample, as follows:

    ______________________________________                                                       5° C.                                                                          23° C.                                          ______________________________________                                        (HC.sub.6 F.sub.12 CO).sub.2 O.sub.2                                                           0.031 hr.sup.-1                                                                         0.19 hr.sup.-1                                     (C.sub.3 F.sub.7 CO).sub.2 O.sub.2                                                             0.043 hr.sup.-1                                                                         0.14 hr.sup.-1                                     ______________________________________                                    

At a temperature of at least 35° C., a half-life period of thepolymerization initiator is about one hour and the concentration of thepolymerization initiator remarkably changes so that such temperature isunsuitable for the uniform polymerization reaction. The polymerizationtemperature is usually selected from a temperature range between 0°and35° C., particularly between 0°and 28° C. The pressure at thepolymerization reaction is determined according to the content ratio ofTFE to HFP in the mixture monomer, the presence/absence of the solvent,and the polymerization temperature. Generally, when the solvent is notused, the reaction pressure is suitably from about 5 to about 15 kg/cm²(an indicated pressure of a gauge, the same hereinafter). Whentrichlorotrifluoroethane and the like are used as the solvent, thereaction pressure varies according to the ratio of the solvent to themonomer and is suitably from about 0.5 to about 15 kg/cm².

As the reaction proceeds, the pressure in the polymerization system isdecreased. In order to compensate the decrease of the pressure, only TFEmay be added so as to continue the reaction. Alternatively, both of TFEand HFP may be added in the amounts corresponding to the amounts of theconsumed monomers. The polymerization time depends on the yield of theobjective copolymer as well as the amount of the used polymerizationinitiator, the ratio of TFE to HFP and the polymerization temperature.The polymerization time is usually selected from the time range between2 hours and 100 hours. The presence of oxygen during the polymerizationreaction is not preferable.

When the yield of the copolymer reaches the desired value, a crudeTFE/HFP copolymer is obtained in the form of from fine particles togranules by recovering the monomer.

The TFE/HFP copolymer obtained by the method of the present inventionhas many advantages such as the advantage that it has an unchangedmolten viscosity, is stable and has no black discoloration in the; stepof the melt extrusion or in the step of thermally shaping the pelletizedmaterial after the extrusion (in addition, the method of the presentinvention is commercially remarkably more advantageous than thereactions in the solvent, since the former is the reaction in water).The method of the present invention can be advantageously used for themolded material and the like.

PREFERRED EMBODIMENT OF THE INVENTION

The present invention will be illustrated by the following

EXAMPLES Example 1

In a jacketed glass-lined autoclave having a stirrer which can receive7,500 parts by weight of water, demineralized deaerated pure water(2,000 parts by weight) and ω-hydroperfluoroheptanoic acid (afluoroalkylcarboxylic acid) (1.0 parts by weight) were charged. Theatmosphere in the autoclave was fully replaced with a pure nitrogen gas,and the nitrogen gas was removed. HFP (2,000 parts by weight) wasinjected in the autoclave and then TFE was injected so that the internalpressure in the autoclave was 8.3 kg/cm² G. The temperature was adjustedat 25.5° C. and the stirring was initiated.Di(ω-hydrododecafluoroheptanoyl) peroxide was added as a polymerizationinitiator and methanol was added as a molecular weight controllingagent. The reaction was initiated immediately. During the reaction, TFEwas gradually added according to the decrease of the pressure so as tomaintain the constant pressure. After the reaction was conducted for56.8 hours, the monomer was purged. A resultant polymer was separated,washed and dried to give a granular polymer.

The conditions of the polymerization reaction, and the yield andproperties of the polymer are shown in Table 1. A specific meltviscosity of the polymer was measured as follows:

The specific melt viscosity means an apparent melt viscosity which ismeasured at 372° C. under a shear strength of 0.475 kg/cm². Namely, thisvalue is measured by a melt indexer. The polymer is filled in a cylinderhaving an internal diameter of 11.4 mm and kept at 372° C. for 5minutes. The polymer is extruded through an orifice having an internaldiameter of 2.0 mm and a length of 8 mm under a piston load of 5 kg. Thespecific melt viscosity is calculated on the basis of the extrusionspeed (MI (g/10 min.)) according to the following equation:

    Specific melt viscosity=10.sup.[(1.618-Log(MI))/0.980+4]  [poise]

Comparative Example 1

The copolymerization of TFE and HFP was conducted in the same manner asin Example 1, except that the amounts of the used raw materials and thereaction conditions are shown in Table 1. The yield and properties ofthe resultant polymer are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                       Comp. Ex. 1                                                                             Example 1                                            ______________________________________                                        Amounts of raw materials                                                      Amount of used water                                                                           2,000       2,000                                            [parts by weight]                                                             Amount of charged HFP                                                                          2,000       2,000                                            [parts by weight]                                                             Pressure of charged TFE                                                                        8.30        8.30                                             [kg/cm.sup.2 G]                                                               Fluoroalkylcarboxylic acid                                                                     0           1.0                                              [parts by weight]                                                             NaHCO.sub.3      1.25        0                                                [parts by weight]                                                             Amount of charged methanol                                                                     First time  First time                                       [parts by weight]                                                                              13.4        11.7                                                              Second time Second time                                                       13.4        11.7                                                              Third time  Third time                                                        13.4        11.7                                                              Fourth time Fourth time                                                       13.4        11.7                                             Amount of charged                                                                              First time  First time                                       polymerization initiator                                                                       2.85        2.0                                              [parts by weight]                                                                              Second time Second time                                                       2.85        2.0                                                               Third time  Third time                                                        1.13        1.13                                                              Addition    Addition                                                          0.19        0.19                                             Conditions                                                                    Temperature [°C.]                                                                       25.5        25.5                                             Reaction time [hr]                                                                             58.0        56.8                                             Polymer                                                                       Yield [parts by weight]                                                                        2,500       2,500                                            Specific melt viscosity [poise]                                                                6.5 × 10.sup.4                                                                      6.5 × 10.sup.4                             Melting point [°C.]                                                                     269         266                                              ______________________________________                                    

The pH change during the copolymerization is shown in FIG. 1. In theComparative Example, the pH remarkably changes at the beginning of thecopolymerization. But, in the Example, the pH is stable.

The results of a metal content of a shaped article after the heatmelting are shown in Table 2. Table 2 shows that the metal content islow in Example 1.

                  TABLE 2                                                         ______________________________________                                                    Comp. Ex. 1                                                                            Example 1                                                ______________________________________                                        Iron ion      23         4                                                    Nickel ion     0         2                                                    Sodium ion    640        67                                                   ______________________________________                                         Unit: ppb                                                                

Since, according to the method of the present invention, the hydrolysisof the initiator at the beginning of the polymerization is prevented andthe initiator is effectively used, the amount of the initiator at thebeginning of the polymerization can be decreased by at least 30%. Thetotal amount of the catalyst can be decreased by, for example, at least20 or 30%. The pH is constant from the beginning to the end of thepolymerization so that the copolymer having a constant molecular weightcan be prepared with high reproductivity. Since the polymerizationvessel is less corroded during the polymerization, the metal content ofthe polymer is low. In addition, the polymer has low discoloration afterthe heat melting.

What is claimed is:
 1. A method for preparing atetrafluoroethylene/hexafluoropropylene copolymer by polymerizingtetrafluoroethylene and hexafluoropropylene in an aqueous medium using,as a polymerization initiator, a di(fluoroacyl) peroxide of the formula:##STR3## wherein X¹ and X² each is a hydrogen, fluorine or chlorineatom, m is an integer of 4 to 8 when X¹ is the hydrogen atom, and m isan integer of 3 to 8 when X¹ is the fluorine or chlorine atom, and n isan integer of 4 to 8 when X² is the hydrogen atom, and n is an integerof 3 to 8 when X² is the fluorine or chlorine atom, characterized inthat a fluoroalkylcarboxylic acid of the formula: ##STR4## wherein Y¹ isa hydrogen, fluorine or chlorine atom, and p is an integer of 4 to 8when Y¹ is the hydrogen atom, and p is an integer of 3 to 8 when Y¹ isthe fluorine or chlorine atom, is added to a polymerization reactionbefore the initiation of the polymerization or during thepolymerization.
 2. The method according to claim 1, wherein the amountof the fluoroalkylcarboxylic acid is from 0.01 to 1.0 parts by weightper 100 parts by weight of water.
 3. The method according to claim 1wherein the fluoroalkylcarboxylic acid is selected from the groupconsisting of ω-hydroperfluoroheptanoic acid, ω-hydroperfluorononanoicacid, ω-hydroperfluoropentanoic acid, perfluoropropionic acid,perfluorobutanoic acid, perfluoropentanoic acid, perfluorohexanoic acid,perfluoroheptanoic acid, perfluorooctanoic acid and perfluorononanoicacid.
 4. The method according to claim 1 wherein the di(fluoroacyl)peroxide is selected from the group consisting ofdi(ω-hydro-dodecafluoroheptanoyl) peroxide,di(ω-hydro-tetradecafluorooctanoyl) peroxide,di(ω-hydro-hexadecafluorononanoyl) peroxide, di(perfluorobutyryl)peroxide, di(perfluorovaleryl) peroxide, di(perfluorohexanoyl) peroxide,di(perfluoroheptanoyl) peroxide, di(perfluorooctanoyl) peroxide,di(perfluorononanoyl) peroxide, di(ω-chloro-hexafluorobutyryl) peroxide,di(ω-chloro-decafluorohexanoyl) peroxide,di(ω-chloro-tetradecafluorooctanoyl) peroxide,ω-hydro-dodecafluoroheptanoyl-ω-hydro-hexadecafluoroonoanoyl peroxide,ω-chloro-hexafluorobutyryl-ω-chloro-decafluorohexanoyl peroxide, andω-hydro-dodecafluoroheptanoyl-perfluorobutyryl peroxide.
 5. The methodaccording to claim 1 wherein the Y¹ --C_(p) F_(2p) -- group in thefluoroalkylcarboxylic acid is the same as at least one of the X¹ --C_(m)F_(2m) -- group and the X² --C_(n) F_(2n) -- group in the di(fluoroacyl)peroxide.
 6. The method according to claim 5 wherein the Y¹ --C_(p)F_(2p) -- group in the fluoroalkylcarboxylic acid is the same as boththe X¹ --C_(m) F_(2m) -- group and the X² --C_(n) F_(2n) -- group in thedi(fluoroacyl) peroxide.
 7. The method according to claim 1 wherein thefluoroalkylcarboxylic acid is ω-hydroperfluoroheptanoic acid and thedi(fluoroacyl) peroxide is di(ω-hydrododecafluoroheptanoyl) peroxide.